1. Field of the Invention
The present invention relates to an image forming method and an image forming apparatus, such as a printer and copy machine, based on the simultaneous transfer-fix scheme, in which a toner image formed on a toner image retaining medium is transferred and at the same time fixed onto a recording medium.
2. Description of the Prior Art
The conventional widely-used image forming method is designed to form a static latent image on a photosensitive member, develop the latent image with dry toner, transfer the toner image onto a recording medium, i.e., print paper electrostatically, and fix the toner image on the print paper. This image forming method, however, has a problem of the deterioration of print quality due to the rough surface of the print paper. Namely, the print paper does not come in precise contact with the photosensitive member and the resulting unevenness of gap causes the disturbance of transfer electric field and the emergence of Coulomb's repulsive force among toner particles.
In order to cope with this problem, there have been invented an image forming method for producing a color copy, in which toner images of multiple colors are transferred by being superimposed onto an intermediate transfer medium, the multi-color toner images are fused on the intermediate transfer medium, and the molten multi-color toner image is transferred and at the same time fixed on a recording medium, and an image forming method in which a toner image formed on a photosensitive member having a shape of endless belt is fused, and it is transferred and fixed on a recording medium. These methods do not base the transfer of toner image onto the recording medium on the electrostatic scheme, and accordingly the above-mentioned deterioration of print quality is unlikely to arise.
In regard to the image forming method of this sort, there have been proposed techniques for improving the transfer of a toner image from a toner image retaining medium, such as an intermediate transfer medium or a photosensitive member, onto a recording medium, in which the toner image retaining medium and recording medium in tight face-contact with each other are heated and pressed, these face-contact members are cooled so that toner solidifies, and the recording medium, with the toner image being fixed on it, is separated from the toner image retaining medium, as described in U.S. Pat. No. 2, 990,278, and Japanese Published Unexamined Patent Application Nos. Hei 5-19642,5-107950 and 5-249798. This method is designed to separate toner from the toner image retaining medium after the cohesion among toner particles exceeds the adhesion between toner and the toner image retaining medium, and accordingly it can prevent the occurrence of so-called "offset" in which part of toner remains on the toner image retaining medium. Thus, the method enables the oil-less process and improves the color balance owing to the high toner transfer efficiency. Moreover, based on the toner solidification along the surface of the toner image retaining medium, high-quality prints with the well polishing and transparent features of toner can be produced by using a toner image retaining medium having a smooth surface.
FIG. 15 shows schematically the structure of the image forming apparatus which adopts the simultaneous transfer-fix scheme.
This image forming apparatus has a train of four photosensitive members 1-1, 1-2, 1-3, and 1-4 of the roller type. These photosensitive members are charged uniformly by respective chargers 10-1, 10-2, 10-3, and 10-4, while being turned in the direction indicated by the arrow, and static latent images are formed on the photosensitive members based on the scanning of light beams, which are modulated by the image density signals, from a light beam scanning device 20. The static latent images are developed with toners of black, yellow, magenta, and cyanine by respective developers 11-1, 11-2, 11-3, and 11-4, so that toner images of these colors are formed on the respective photosensitive members.
The apparatus further includes an intermediate transfer medium 50 having a shape of endless belt which circulatively moves along the four photosensitive members 1-1, 1-2, 1-3, and 1-4. The intermediate transfer belt 50 is looped to run around rollers 5-1, 5-2, 5-3, and 5-4 and a heat roller 2, with a certain magnitude of tension being applied thereto by the tension roller 5-3, and move along the respective photosensitive members 1-1, 1-2, 1-3, and 1-4, so that the multi-color toner images formed on the photosensitive members are transferred by being sequentially superimposed on the transfer belt 50 by the operation of respective transfer devices 50-1, 50-2, 50-3, and 50-4.
When the multi-color toner image reaches the heating section A where the intermediate transfer belt 50 runs around the heat roller 2, it is fused by the heat generated by a heat source 120 which is installed inside the heat roller 2. The molten toner image further goes and reaches the transfer-fixing section B. A sheet of print paper 60 is taken out of a paper tray 6, and fed to the transfer-fixing section B by way of a paper feed roller 6-1, guide roller 6-2, and register roller 6-3 by being timed so that the toner image on the intermediate transfer belt 50 comes to the transfer-fixing section B coincidently.
A press roller 3 is disposed at the transfer-fixing section B to confront the heat roller 2, and it operates by being timed to the feeding of the print paper 60 from the paper tray 6 thereby to press the paper to the heat roller 2. Accordingly, in the transfer-fixing section B, the intermediate transfer belt 50, with the toner image being formed on it, and the print paper 60 in face-contact with each other are heated and pressed between the heat roller 2 and the press roller 3. The face-contact print paper 60 and intermediate transfer belt 50, with the toner image being caught between them, go out of the gap between the heat roller 2 and the press roller 3, and reach a cooling device 4, by which toner is cooled to solidify. The paper 60 and belt 50 reach a separating roller 5-4 having a small diameter, on which the print paper 60 having the toner image separates based on its own stiffness from the intermediate transfer belt 50. The separated print paper 60, with the toner image being fixed on it, is carried by a paper carrying device 7 and delivered to the outside of the image forming apparatus.
However, image formation by the foregoing image forming apparatus occasionally incurs an inappropriate phenomenon. Namely, on the print paper 60 after the heat-pressing operation, there emerge numerous creases 61 in the direction perpendicular to the paper carrying direction as shown in FIG. 16, causing toner to peel off the intermediate transfer belt 50 before it is cooled to solidify. This phenomenon not only creates creases on the print paper 60, but deteriorates the print quality considerably. FIG. 17 shows the cross section along the paper carrying direction of the intermediate transfer belt, toner and print paper in this situation. Specifically, at the creasing portion of the print paper 60, toner 100 peels off the intermediate transfer belt 50 before it is cooled to solidify. On this account, toner 100 does not solidify smoothly along the surface of the intermediate transfer belt 50 in this portion, lowering the polish and resulting in a degraded print quality. The above-mentioned impropriety arising during the transfer-fixing process of a toner image on the print paper by way of the intermediate transfer belt occurs similarly in the case of transferring and fixing a toner image directly from a photosensitive member of the belt type, for example, to the print paper.